In image formation based on electrophotographic technology, light information is applied to an electrostatic charge image carrier containing a photoconductive substance in response to the images information by various methods, and an electrostatic charge image is formed on the aforementioned electrostatic charge image carrier. Then the electrostatic charge image is developed as a toner image by charged toner and this toner image is transferred onto a recording medium such a paper. The image is then fixed by heat, pressure or solvent vapor in the fixing process, whereby a visible image is obtained.
In the formation of a full color image based on the aforementioned electrophotographic technology, electrostatic charge images are formed on an electrostatic charge image carrier with the color toner of yellow, magenta, cyan or black by the image information broken down into each color. These images each are developed to get each color toner image corresponding to each image information. These images are then superimposed, transferred and fixed in the fixing step, whereby a full color image is obtained.
Such color toner is exemplified by the yellow toner, magenta toner, cyan toner and black toner formed from toner particles prepared via dispersion of each of yellow, magenta, cyan and black colorant. The conventionally known organic pigment and dye have been used as the colorant used in color toner, and they each have various defects.
For example, the organic pigment is generally superior to dye in heat resistance and light fastness. However, the organic pigment is dispersed in the form of coagulated particles in the binder resin, and yet its dispersibility is low. This will increase the opacifying power of toner and reduces the transparency. In the image having been formed, the undermost layer of the color toner layers of each of colors having been superimposed will be covered by the upper layers, with the result that the visibility of the toner color of the undermost layer, chromaticness and the image color reproduction are reduced. In principle, all colors can be reproduced by the subtractive color mixture process using the three primary colors of yellow, magenta and cyan. However, when a color image is formed by the color toner including the pigment, the color, range and chromaticness to be reproduced may be limited in practice by the spectral characteristic at the time of dispersing the pigment in the binder resin, and the color mixture at the time of mixing the toner of different colors. This may lead to a failure in ensuring a faithful reproduction of the document color.
To solve the aforementioned problems of the pigment, the flashing method for dispersing the pigment is used. This method makes it possible to achieve the size of the particle dispersed on the order of submicron by primary particles free of coagulated secondary particle. The technique of improving the transparency using this method has been proposed. Another technique having been proposed is the one for improving the electrostatic charging property, fixing performance and image uniformity by covering the particle-like pigment with the binder resin and outer-shell resin.
However, the aforementioned means fails to the meet the requirements of ensuring the sufficient transparency and chromaticness of the toner containing pigments as colorants.
On the one hand, proposed examples of the toner using a dye exhibiting excellent color reproduction and transparency as a colorant include toner using an oil-soluble dye, toner using a colorant in which a reactive dye adheres to silica, toner using a colorant strengthened by a polymer dye or such, and toner using a colorant mixing a pigment and a dye. However, the dye is generally present as it is dissolved in the binder resin for toner particle formation constituting the toner particles, and therefore exhibits excellent transparency and chromaticness. For the characteristics thereof, the light fastness and heat resistance are far inferior to those of the pigment. Since it is interior in heat resistance, the dye will be decomposed by heat and image density will be reduced. Further, when the toner image is to be fixed by a contact heating process, the dye may be sublimated by heat, with the result that the apparatus is contaminated. Furthermore, the dye may be dissolved into the silicone oil used at the time of fixing, and may be transferred fused onto the heating roll in the final phase, so that offset phenomenon occurs. This problem has actually taken place.
A technique having been proposed to solve the aforementioned problem with the dye is exemplified by a method of ensuring compatibility among light fastness, heat resistance and color reproduction by using a specific anthraquinone based dye. However, the aforementioned method is not yet powerful enough to provide toner exhibiting sufficient heat resistance (sublimation property) and light fastness.
Another technique having been proposed to solve the foregoing problem with the dye is also exemplified by a method for producing the encapsulated toner, wherein the core particle containing the polymer resin and colored dye is covered by a shell layer polymer (refer to Patent Document 1, for example).
However, this technique is still insufficient to provide toner exhibiting sufficient heat resistance and light fastness.
As a technique proposed to solve a problem with the dye, there is a method of producing toner in which a black dye and an antioxidant are dispersed in a solvent to prepare colored particles by adding polymerizable particles, in order to avoid color fading or discoloration (refer to Patent Document 2, for example).
However, after dispersing or dissolving constituent materials of polymerizable particles in a solvent, a process of removing the solvent or such is desired to be conducted in order to acquire sufficient light fastness by impregnating a dye and an antioxidant inside the polymerizable particles, and also the simple coexistence of a dye and an antioxidant, together with polymerizable particles result in the slight deposit amount adhering on the surface of polymerizable particles. Since the dye and the antioxidant are not also present in a strong interaction, the antioxidant effect tends not to be obtained effectively. It is also difficult to obtain sufficient heat resistance.
After dispersing or dissolving in an organic solvent a resin and a dye which are possible to be reacted with each other, disclosed is a method of removing a solvent to prepare a toner by reacting these resin and dye (refer to Patent Document 3, for example).
However, in the case of a toner disclosed in Patent Document 3, insufficient light fastness is possibly obtained, since the antioxidant relating to the toner storing for a long duration tends to be deactivated, though a dye is fixed to a resin, whereby high heat resistance is obtained. There is also a problem such that a charging characteristic between toner particles is extremely uneven, since a dye is to be present on the toner particle surface.
(Patent Document 1) Japanese Patent O.P.I. Publication No. 5-72792
(Patent Document 2) Japanese Patent O.P.I. Publication No. 6-148928
(Patent Document 3) Japanese Patent O.P.I. Publication No. 2004-77707